Recently, to achieve higher energy saving in image forming apparatuses of an electrophotographic system, there is a need for a toner for electrostatic image development (hereinafter may be referred to simply as a “toner”) that is heat-fixable at lower temperature. Such a toner is required to have higher low-temperature fixability and also have long-term stable charge property so that high quality images can be formed for a long period of time.
For example, Patent Literature 1 discloses a toner containing a crystalline polyester resin as a fixing aid.
However, with such a toner, the following problems occur unless the compatibility between the crystalline polyester resin and a binder resin is taken into consideration. For example, when the compatibility between the crystalline polyester resin and the binder resin daring heat fixation is high, there arises a problem in that heat-resistant storage stability is low because plasticization of the binder resin proceeds before heat fixation. On the ether hand, when the compatibility between the crystalline polyester resin and the hinder resin is low, there arises a problem in that sufficient low-temperature fixability is not obtained. Another problem in this case is that the crystalline polyester resin is separated and exposed at the surface of toner particles to cause a reduction in the charge property of the toner, so that image failures such as a reduction in image density and fogging occur.
Therefore, Patent Literature 2, for example, proposes that the affinity between the binder resin and the crystalline polyester resin and the concentration of ester groups in the crystalline polyester resin are controlled to achieve both the low-temperature fixability and long-term stable charge property simultaneously.
However, Patent Literature 2 has a problem in that, although low-temperature fixability and heat-resistant storage stability acre achieved, the long-term stability of charge property is low. This is because, although the compatibility between the crystalline polyester resin and the binder resin is taken into consideration, the binder resin used in combination with the crystalline polyester resin is also a polyester resin, i.e., resins of the same type are combined, another problem is that it is not possible to completely prevent the crystalline polyester resin from being exposed at the surface because, although the compatibility is controlled, these resins are of the same type.
Patent Literature 3 proposes that a combination of a crystalline polyester resin and a copolymer, obtained from a styrene-based monomer and a (meth)acrylate-based monomer is used as the binder resin. To achieve low-temperature fixability and heat-resistant storage stability simultaneously, the surface of the crystalline polyester resin used as seeds is surrounded by the resin of the same type as of the binder resin. However, there is no study on compatibility to completely prevent the crystalline polyester in the binder resin from being exposed at the surface, and there still arises a problem in that the long-terra stability of charge property is low.